This invention generally relates to control systems and methods for three-phase pulse-width-modulated (PWM) alternating-current (AC) voltage regulators (also referred to as AC choppers). More particularly, the present invention relates to a system and method for implementing a set of four-quadrant switches in a pulse-width-modulated alternating-current voltage regulator based on commutation of the load currents among different converter switches.
A number of applications, such as air-conditioning or refrigeration applications, utilize multi-phase induction motors. The starting, or inrush, current for multi-phase motors tends to be several times the rated full-load current. This high inrush current may have many detrimental effects on the equipment and the power system in general, as well as the economics of power usage. By means of example only, drawing high inrush current over a long power line may cause the voltage to essentially collapse, leaving insufficient voltage for equipment to operate. Furthermore, with high motor inrush current, other customers along the same power line may experience undesirable voltage fluctuations during the start of the motor. To discourage this situation, power companies sometimes impose penalties if a customer""s starting or inrush currents are excessive. This is particularly true in regions with xe2x80x9cweakxe2x80x9d power grids, such as Europe. Thus, it is desirable to minimize the current drawn by a multi-phase induction motor during starting.
Several known methods exist which allow for the reduction of induction motors"" inrush current. Use of an autotransformer is one known method for achieving lower motor starting currents. Autotransformers, however, are relatively inflexible in that the turns ratio of an autotransformer is established up front and remains fixed by the design of the components. Another approach employs the use of series elements such as inductors, resistors, and the like, to limit starting current. The latter approach, however, requires significantly higher line currents than autotransformer starters to provide the same amount of torque. Yet another approach consists in employing the so-called wye-delta motor starters. This type of equipment configures the connection of induction motor windings in a different manner during the motor start-up than during the regular motor operation. This allows the motor to start with a reduced inrush current.
The above methods for achieving reduced motor inrush currents can all be characterized as electro-mechanical methods. They require a set of electro-mechanical contactors in order to alter the connection of an induction motor to the power line. This altering of connection further results in a reduced voltage being applied to each of the motor""s windings, which in turn results in reduced inrush currents. Electro-mechanical contactors have the disadvantage of being expensive and prone to reliability problems due to wear and tear. In addition, their transitions can cause voltage or current spikes with potentially damaging effects to the system.
The problems associated with electro-mechanical starting methods for induction motors can be avoided by employing electronic (or solid-state) starting methods. Electronic motor starters reduce the voltage supplied to an induction motor during its startup by means of a power electronics converter. One such converter technology employs thyristors, also called silicon-controlled rectifiers (SCRs). SCRs are semiconductor switches that can be turned on by means of an electronic signal. However, they cannot be forcefully turned off, i.e. they can be turned off only if the current through them naturally extinguishes itself. In a typical SCR-based motor starter, two SCRs are back-to-back connected between each of the motor""s phases and the power line. During the motor start-up, the SCRs are turned on only once in every line cycle, and this is done in a delayed fashion, so that the motor is actually connected to the power line for only a portion of each line period. This results in a reduced voltage being applied to the motor, and therefore a reduced inrush current being drawn from the power line. The amplitude of the fundamental voltage being supplied to the motor is controlled by the time instant when an SCR is turned on within a line cycle. This type of control is usually referred to as phase control.
With no electro-mechanical contactors needed, SCR-based solid-state motor starters represent an improvement over electro-mechanical starters in terms of reliability and cost. However, SCR-based electronic starters have the disadvantage of distorting motor""s current and voltage waveforms during the start-up. In addition, they offer no possibility for improving the power factor (power factor is intended as the phase displacement between the fundamental component of voltage and current at the line terminals feeding the motor starter). A good power factor is generally a desired feature in any electrical system.
Alternatively, PWM AC voltage regulators can be used for starting large induction motors, as they allow for a significant reduction in inrush current and provide better quality motor current and voltage waveforms during start-up than SCR-based motor starters. Similar to SCR-based technology, a PWM AC voltage regulator includes power electronics converter capable of supplying an output voltage of a fixed frequency, but at a variable magnitude, to AC loads. PWM AC voltage regulators differ from phase-controlled SCR-based AC voltage regulators, in that, with PWM AC voltage regulators, the switching of power semiconductors occurs at a frequency (called the switching frequency) many times higher than the input line frequency (usually equal to 50 or 60 Hz). Such high rate of semiconductor switching can be achieved with modern power semiconductors with full turn-on and turn-off capability, such as, for example, insulated gate bipolar transistors (IGBTs). The control of the fundamental amplitude of the output voltage of a PWM AC voltage regulator is achieved through the control of the width of the pulses of which the output waveform in such a regulator consists. A single-phase PWM AC voltage regulator circuit is described in U.S. Pat. No. 5,923,143 to Cosan et al. entitled xe2x80x9cSolid State Motor Starter with Energy Recovery.xe2x80x9d
PWM AC voltage regulators, when used for starting of induction motors, have several advantages compared to SCR-based motor starters. First, they are able to start a motor with a smaller fundamental component of line inrush current. Typically, if an SCR-based motor starter requires an inrush current equal to 45% of motor""s locked-rotor current (LRA), a PWM AC voltage regulator used with the same motor shall require around 20% of LRA. Second, PWM AC voltage regulators generate better-quality motor current and voltage waveforms during the start-up. This results in lower pulsating torque produced by the motor, which, in turn, benefits the motor""s mechanical driveline. Finally, PWM AC voltage regulators offer the possibility for power factor correction.
Practical implementation of three-phase PWM AC voltage regulators requires semiconductor switches that can conduct, or block, electric current flow in either direction in a fully controllable manner. Such switches are also referred to as four-quadrant switches. No such single semiconductor device is available commercially nowadays. Therefore, four-quadrant switches are implemented as a combination of two two-quadrant switches (i.e. switches that can fully control the current flow in one direction only). Examples of two-quadrant semiconductor switches are bipolar junction transistors (BJTs), gate turn-off thyristors (GTOs), field-effect transistors (FETs) and previously mentioned IGBTs. When both two-quadrant switches in a four-quadrant switch are turned on, the four-quadrant switch conducts electric current in either direction. When both two-quadrant switches are turned off, the four-quadrant switch blocks electric current from either direction. When only one of the two two-quadrant switches is turned on, the four-quadrant switch conducts current only in the direction allowed by the two-quadrant switch that is turned on.
Control of a three-phase PWM AC voltage regulator may be accomplished in different ways. Two possible methods are described in A. Mozdzer and B. K. Bose, xe2x80x9cThree-Phase AC Power Control Using Power Transistors,xe2x80x9d IEEE Transactions on Industry Applications, Vol. IA-12, No. 5, September/October 1976, pp. 499-505 and in S.A.K. Bhat, xe2x80x9cDigitally-controlled multiple-pulse-width-modulated AC chopper for power control,xe2x80x9d International Journal of Electronics, Vol. 51, No. 1, 1981, pp. 45-56. Both control methods are developed for four-quadrant switches implemented with BJTs. They are both dependent on the load power factor, which makes them unsuitable for starting of induction motors (during the start-up, the power factor of a typical induction motor changes from a low value to a high value). In addition, under certain operating conditions, the described methods result in distorted output voltage and current waveforms. There is therefore a need for a control method for three-phase PWM AC voltage regulators suitable for starting induction motors with good voltage and current waveforms.One typical AC-to-AC converter topology that requires four-quadrant switches is a matrix converter. In a matrix converter, each phase of a polyphase input is connected, via a four-quadrant switch, to each phase of a polyphase output. A control method for four-quadrant switches, as applied to matrix converters, is described in N. Burany, xe2x80x9cSafe Control of Four Quadrant Switches,xe2x80x9d 1989 IEEE IAS Annual Meeting, pp 1190-1194. As detailed in this reference, the commutation of a load current in a matrix converter always occurs between two four-quadrant switches connected to the same load terminal. Each of these four-quadrant switches connects the load current to an input voltage source. The commutation is carried out in four consecutive steps where each step includes turning on, or off, one of the four available two-quadrant switches. The two-quadrant switches to be turned on (or off) are chosen, as is the sequence in which the switches are turned on and off, in such a way that the input voltage sources are never shorted (shorting them would cause a potentially destructive over-current) and the inductive load current is never interrupted (interrupting it would cause a potentially destructive over-voltage). The time interval between any two consecutive steps is long enough to allow for the two-quadrant switches to turn completely on (or off). Due to the matrix converter topology, the commutation of any one of the three load currents is independent from the other two. Thus, the commutation sequence, as explained in the above-referenced paper, is valid for both single- and polyphase matrix converters.
The topology of a three-phase PWM AC voltage regulator differs from that of a matrix converter. Specifically, commutation of any one of the three load currents is not independent from the other two. Consequently, the commutation method described above is not acceptable for applications of this type.
The present invention provides a system and method for implementing and controlling a set of four-quadrant switches in a PWM AC voltage regulator. This system and method does not result in a distortion of output current and voltage waveforms, and is independent of the load power factor. In addition, by providing a commutation method for four-quadrant switches, it is possible to provide a system to control a PWM AC voltage regulator without short-circuiting input voltage sources or interrupting load currents. The present invention provides a control method for commutating the load currents among different converter switches at specific time instances without causing short-circuit of the converter input voltage sources or interruptions of the inductive load currents.
To attain the advantages and in accordance with the purposes of the invention, as embodied and broadly described herein, there is provided a method for implementing and controlling a three-phase pulse-width-modulated alternating current voltage regulator to convert a commercial three-phase input to a variable three-phase output to be supplied to a load, wherein the regulator includes six four-quadrant switches, each four-quadrant switch comprising two two-quadrant switches. Three series branches, each series branch including one four-quadrant switch, and three shunt branches, each shunt branch including one four-quadrant switch, are provided. The polarity of a current being applied to the load is sensed. To obtain a desired converter state, the two quadrant switches in the series and shunt branches are turned on or off in a predetermined sequence based on the sensed polarity of the current being applied to the load. Further, there is provided a three-phase, PWM AC voltage regulator system for converting a commercial three-phase input to a variable three-phase output to be supplied to a load, wherein the system includes a three phase voltage source, a converter circuit, a three phase load, load current sensors, a system for sensing the polarity of a current being applied to the load, and a processor. The converter circuit comprises six four-quadrant switches, each four-quadrant switch being implemented by two two-quadrant switches. The processor is configured to obtain current polarity information and to turn the switches on and off in a predetermined manner to obtain the desired voltage based on the sensed polarity of the current being applied to the load.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be appreciated by one of ordinary skill from the description, or may be learned by practice of the invention. The objects and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in this application and the appended claims.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.